The 2X4 brick was introduced
in 1949 and is one of the two original LEGO bricks..

The first type (2X4vs01) is
illustrated above (but see also this alternate
candidate to the honor of being the first type). There are no inscriptions
on the bricks (logo or mold number) and thus they may be confused with
unbranded products of other origins.

When seen from below, the inside
corners are very rounded. This is an important distinguishing feature.

There were only mold units
for two types of bricks: with a single lateral slot; and with two opposing
lateral slots. The mold had at least five units: three for double slotted
bricks and two for single slotted bricks. These may be told apart because
the molding pip is on opposite sides of the slot.

In the first issues (from 1949
to 1951?) the bricks were molded with a distinctive catalin-type resin
not used subsquently. They do not warp, are very shiny, and have a milky
aspect and, as in all plastics of this type, they show considerable transparency
against a soft light, even the white coloured bricks.

NOTE: In LEGO bricks the
slots are molded and not cut and thus the same molding unit always
produces the same type of slotted brick.

When a new mold was needed
The brand name "LEGO" was conspicuously carved in the mold.
This may suggest a need to diferentiate from immitators. Since the10X20
thin baseplate issued in 1951 was not similarly marked, this mold is more
recent.

No mold unit numbers are engraved.
Seen from below, the corners are only slightly rounded (much less than
vs01). Looking from beneath into the brick interior, thin longitudinal
ribs will be noticed. These were probably carved in the mold to help with
the ejection process.

Bessides 2X4vs01, this is the
only other mold used for 2X4 colours that had been abandoned by 1953,
such as light green and marbleized. A comparative study of colours and
plastic characteristics shows that the mold 2X4vs01 was not abandoned
at this time and both molds were used simultaneously.

Molds were made at this time
again to produce only the single slotted bricks and the opposed slotted
bricks. The first 3X4 mold was produced at the same time as this.

This is a different mold from
the previous . Seen from beneath, the interior ribs are transversal and
not longitudinal (they are clearly seen on one of the sides of the interior
of the red, the blue and the yellow bricks).

This
mold had two life-phases. Originally the letters of the "LEGO"
logo were less marked and the molds were not numbered. Subsequently the
LEGO logo was deepened and molds were hand inscribed so that each brick
was marked with a faint mold number (the red brick in the figure above
is "3", just visible upside down in the lower part of the brick).

For the first time with this
mold, the four slot combinations were produced. The mold numbers according
to type were:

Circa 1954 a new mold was produced.
Bricks were still slotted and mold units were made to produce all four
slot combinations. The main difference is now the so-called LEGO "dogbone"
logo engraved inside.

Seen from above, the sides
of some studs are seen to be taller than the top- see the two diametrally
opposed studs of the white brick in the picture above.
This is a result of using ejector pins on the cavity side of the mold,
one over each of the two studs. The marks are seemingly caused by an intrusion
of plastic on the periphery of the ejector pin tops.

Machine-engraved neat mold
numbers were used (partially seen on top of the yellow and red bricks
above).

Mold numbers used:

-Nr 1 and 2 - single slotted;

-Nr 3 and 4 - slotted at 90º
as the yellow brick above;

-Nr 5 and 6 - slotted at 90º
as the red brick above;

-Nr 7 and 8 - two opposing
slots.

NOTE: The very first boxed
sets sold in Germany in the Winter of 1955/56 had this kind of bricks
in red and white.

The problem with the concavity
of the stud surfaces on high speed injection molding, seen e.g. on the
1X2 bricks, called for thinner stud walls, and so in the first molds for
non-slotted bricks produced by LEGO, the studs were hollowed.

Another feature was introduced
in all the new molds: the "LEGO" logo on the studs.

There were at least two molds
made, but their differences do not justify type status. In one the mold
unit numbers are printed at the top; in the other they are printed at
the center but from right to left (as if seen in a mirror).

Mold
numbers seen: top printed- "1" to "19" and there are
also unnumbered bricks; center printed- "1" to "18".

Bricks of this type were certainly
sold in 1956-57 in Scandinavia and also in Germany in 1956, but were not
seen in sets sold in Portugal.

NOTE: Blue and yellow bricks
are unusual and green must be quite rare- I only know it from an image
sent by fellow collector Maxx Kroes.